1. Field of the Invention
The invention relates generally to control valve systems and more particularly, it concerns a control valve system for opening and closing blowout preventers.
2. Description of Prior Art
Two major classes of blowout preventers are currently utilized to shut off uncontrolled flow of pressurized fluid in applications such as oil and gas wells--ram-type blowout preventers and spherical blowout preventers. In the operation of a spherical blowout preventer, a working fluid is injected on the closing side of a built-in piston to force the piston against a flexible closure element, thereby expanding the element into the flow path to cut off flow. In a ram-type blowout preventer, a hydraulic cylinder having a rod attached to its piston is utilized to move a ram, which acts as the closure element to close the passage of the pressurized fluid. While the discussion, below, focuses on the latter class of blowout preventers, it should be apparent to those of skill in the art that it applies equally to spherical blowout preventers.
Cylinder-piston-and-rod operator devices (operator cylinders) have long been utilized as operators for blowout preventers. These devices generally include a closed cylinder with a piston, slidably mounted inside the cylinder, and a rod, secured to the piston and extending out of one end of the cylinder. The piston and cylinder, therefore, had a blind side and a rod side as designated by the location of the rod.
In the past, these operators functioned hydraulically by injecting fluid into the cylinder on the blind side in order to move the piston and rod to an extended position so that the rod operated the blowout preventer closure means to close off flow from the well. Fluid contained in the cylinder on the rod side of the system was, in turn, vented back to a reservoir upon the motion of the piston to the end of the cylinder from which the rod extended. Such operation demanded great volumes of fluid to move the piston and rod from a fully open position to a fully closed position. Moreover, since installed horsepower, i.e., the horsepower required to fully move the piston, is equal to the volumetric flow through the pump multiplied by the pressure in the lines, this large fluid requirement also created a large horsepower requirement on the pumping device used to move the fluid.
Accordingly, many attempts have been made to reduce the horsepower and fluid requirements of such an operator. In U.S. Pat. No. 3,360,807 to Lucky, a valve apparatus is illustrated which is designed to utilize the downhole pressure created by a blowout to aid in closing the blowout preventer. This device is believed to be disadvantageous, however, in that it uses whatever fluid or substance may be downhole as its driving fluid. Hence, drilling mud or other fluid with suspended debris is circulated through the valve. Such driving fluid is believed to present the disadvantage of potentially clogging the valve mechanism thereby preventing effective operation of the apparatus.
In U.S. Pat. No. 3,299,957 to O'Neil, a fluid control system is shown in FIG. 18 comprising an accumulator cylinder utilized in conjunction with the pump means. In particular, the pump means is continuously operated to effectively raise the piston and pressurize the accumulator. The purpose of this system, however, is to allow the use of a lower horsepower input pump rather than to minimize overall horsepower requirements and fluid requirements. In fact, fluid expelled from the pistons during the lowering motion of the pistons is exhausted to a liquid reservoir each time the pistons are lowered. Hence, the control valve system illustrated in O'Neil apparently utilizes greater fluid and greater installed horsepower than normal systems.
Other systems utilizing accumulators are shown in U.S. Pat. No. 4,098,341 and U.S. Pat. No. 3,044,481. All of these systems are believed to utilize greatly excessive amounts of hydraulic fluid thereby increasing the amount of horsepower required for operation.
Other attempts are believed to have been made to reduce the overall horsepower requirements, but these have involved costly modifications to the blowout preventer structure.
Hence to provide an improved control valve system, it is necessary to provide a system requiring less power to operate the system while also minimizing hydraulic fluid requirements on the system.